<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <!-- 普通的元素 -->
  <script>
    var titleEl = document.querySelector('.title')
    alert(titleEl) // null
  </script>

  <h1 class="title">我是标题</h1>
  <p>我是段落,哈哈哈哈</p>

  <!-- 普通的元素 -->
  <!-- 一般放在最后 -->
  <script>
    var titleEl = document.querySelector('.title')
    console.log(titleEl);
    alert(titleEl)
  </script>

</body>

</html>